Wood-burning stoves have been available for centuries. Perhaps the most well-known wood-burning stove is the Franklin stove which, while being practical for its time, burned wood inefficiently. That stove, like most cast iron stoves available today, provides for updraft combustion, such as is found in a fireplace and in which the volatile gases (volatiles) which are driven off as the wood burns are generally left unburned. The unburned volatiles remain for two reasons, first because the gases, by the time they have left the wood, are generally too cool for secondary combustion and second, because oxygen that is admitted to the stove or fireplace is usually consumed by the coals at the base of the fire mass, causing the gases to rise through an oxygen-deficient atmosphere. The loss of the volatile gases is a serious problem, because they represent approximately half of the total heat value of the wood. It is as though one were to run an open line of natural gas up a chimney without bothering to ignite it first. In addition, the volatile gases given off from the wood without being ignited may also condense on the cool sides of long metal flue pipes and drip out as creosote which may sometimes be inadvertently and dangerously burned, in their sold creosote form, as a chimney fire.
One effective method and apparatus for reducing the volatiles is found in a horizontal combustion heating apparatus wherein the flames move horizontally in the primary combustion zone. This is radically different from typical updraft combustion and is the basis of the more efficient operation of a stove manufactured by Vermont Castings, Inc., the assignee of this invention, which has been sold since at least the first quarter of 1976 under the trade name "DEFIANT". The DEFIANT parlor stove aids the burning of the volatile gases in several ways. First, by using horizontal combustion, the gases are forced to pass close to the hot coals which maintain sufficiently high temperatures to ignite them. In addition, a manually controlled secondary air source, which is segregated from primary air source, provides air which is ducted down a tube integral with the heated fireback of the primary combustion chamber, which is heated by the hottest part of the fire on two sides, and this channel preheats the air to maintain the air at the elevated temperatures required for combustion. Thus, oxygen from the secondary source is led into the secondary combustion chamber through numerous air ports to mix with the combustible gases and to provide secondary combustion. In the "DEFIANT", the secondary source of air is provided in the secondary combustion chamber along an outside wall of the chamber, away from the primary combustion chamber.
In addition, behind the fireback which is provided at the back of the primary combustion chamber, a circuitous path is provided by smoke baffles. The circuitous path is comprised of a plurality of smoke passages, which conduct the smoke through the passages back and forth along the back of the apparatus and upwardly toward the exit at the flue collar. Since the heat of the flue gases is considerable, significant heat transfer occurs from the flue gases to the surfaces of the stove, which in turn is given off into the room rather than being lost up the chimney. In addition, the circuitous path aids in maintaining higher temperature in the combustion chamber which aids in burning the volatile gases driven off from the wood. Thus, in the "DEFIANT", a large heat output is available. In addition, the fire is controlled by not only the structure of the apparatus but by a thermostatically controlled input port which supplies the primary air.
Even though, the "DEFIANT" promotes secondary combustion in the secondary combustion chamber, it is still desirable to increase the efficiency of the unit by whatever means are needed. Thus, while the "DEFIANT" has been hailed as a significant advance in the art of building wood-burning stoves, there should always be room for further improvement.
It is therefore a principal object of this invention to provide a heating apparatus having improved secondary combustion. Further objects of the invention are to provide a wood-burning heating apparatus having a top loading capability and a flexible flue connection.
Yet further objects of the invention are to provide a wood-burning apparatus which is reliable, which provides efficient operation, which is capable of long-term operation with a single load of wood, which has a high heat output, and which can be set for reduced heat output during preselected times.